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Where’s the water vapor?
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| NCAR's S-Pol radar will track water vapor above the Oklahoma Panhandle as part of IHOP2002. (Photo by Carlye Calvin.) |
One of the biggest weather studies in North American history, by some counts, will unfold this May and June. Although its focus is on water vapor, the lead scientists are hoping for clear skies—at least part of the time. This summer’s field campaign of the International H2O Project (IHOP2002), to be based in Oklahoma, is assembling over 100 people, six aircraft, and a unique mix of instruments from the United States and Europe. The main goals are to accurately determine how water vapor varies in three dimensions over time and to use that knowledge to improve warm-season precipitation forecasts.
Where, when, and how hard it will rain are the most difficult forecast elements to nail down, especially in the warm season (and the large overall gap in precipitation forecast skill between summer and winter appears to be growing across the lower 48 states). Improved rain and snow forecasts are one of the main goals of the U.S. Weather Research Program. The USWRP has listed IHOP as a high priority, paving the way for multiagency support. IHOP will combine real-time forecasts with unprecedented high-speed, high-resolution sampling of water vapor from an all-star lineup of moisture sensors.
Unlike many experiments held on the plains during severe-weather season, IHOP won’t be examining showers and thunderstorms themselves as much as the hydrological backdrop that triggers them. “We’re focusing on the moisture content in preconvective situations, when there’s nothing out there,” says IHOP co-lead Tammy Weckwerth (NCAR Atmospheric Technology Division, or ATD). Too many clouds could actually impede some of the lidars and other instruments, so a few clear but humid mornings will be most welcome.
Finding the right mix
One reason so many different sensors are being trained on water vapor is that no single tool does the job. “Ideally, we’d like to have a radar-type display showing water vapor, but we’re far away from that goal,” says Weckwerth. Radiosondes still provide most of the data on water vapor that go into weather and climate models, and even these mainstays have had their problems with accuracy (see sidebar, page 6). Radiosonde launches are also too far apart in both time and space to gather the data needed. Lidars provide more detail; however, they’re limited by clouds and their range is only a few miles. Satellites cover much of the globe, but to date they haven’t provided the type of accurate, high-resolution measurements needed in the lower atmosphere for storm prediction.
Some new arrivals show promise. They include slant-path measurements based on the Global Positioning System (see sidebar, page 4) and radiometers that profile temperature, humidity, and cloud liquid by sensing tiny amounts of molecular radiation. NCAR’s S-Pol radar will assess the atmosphere’s refractivity, thus allowing it to spot horizontal transitions in moisture content. Four of the aircraft in IHOP2002 will carry state-of-the-art remote sensing systems, including passive and microwave sounders and differential absorption lidars. The idea is to replicate current and future satellite systems designed to provide vertical profiles of water vapor, temperature, and winds. “Nearly every available ground-based system from ATD will be in the field for IHOP2002,” says co-lead David Parsons.
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| IHOP2002 co-leads Tammy Weckwerth and David Parsons. (Photo by Carlye Calvin.) |
In mixing these and other sensors at IHOP, the hope is that each kind of instrument can help shed light on the others’ performance—and, in turn, point the way toward an optimal blend that can truly advance prediction. Researchers will assess how the benefits in forecast skill afforded by improved water vapor measurements stack up against potential improvements in other areas, such as better models or wind-field data.
The IHOP sensors will be located in what is already one of the most heavily monitored patches of atmosphere on the planet (see box). The result will be a feast for boundary layer scientists studying the lowest kilometer of the atmosphere. “We’ll also be looking at how surface and boundary layer variations scale upward to affect [thunderstorm] initiation and evolution,” says Parsons. The IHOP aircraft and ground-based mobile units will monitor conditions in and near atmospheric boundaries, including the recurrent dry line, a frontal feature that often serves as a focus for spring storms. Wind data from profilers already in place across the network will be joined by special radiosonde launches and the variety of fixed measurement platforms added for the experiment (see box), including soil moisture stations.
The operational link
There’s a strong modeling and nowcasting element to IHOP. Investigators will be collocated in Norman, Oklahoma, with operational forecasters from NOAA’s Storm Prediction Center. They will examine a suite of models run in near–real time using a subset of readily available data. During the field phase, preliminary data products and operations reports will be available through the Web-based catalog maintained by UCAR’s Joint Office for Science Support (see “On the Web” below). JOSS staff will also take the lead in managing and archiving final data sets, in coordination with the U.S. Department of Energy’s Atmospheric Radiation Measurement project and other participants.
With so many types of water vapor sensors on hand from different sources, ATD investigators will focus on establishing relative accuracies and combining the diverse data into one integrated set. The end product should prove useful in testing and debugging a wide range of models on varying scales.
As for weather forecasts, the ultimate goal is to improve quantitative outlooks of rain and snow. Right now the downpours most threatening to society are still the hardest to predict with precision. When Tropical Storm Allison stalled over Houston in June 2001, some locations got as much as 25 inches of rain (635 millimeters) on a night when official guidance called for amounts on the order of 5 inches (127 mm).
Parsons feels a comprehensive look at the water vapor that fuels heavy rain is “science waiting to happen in a number of research areas”—many of them to be addressed through NCAR’s new multiscale strategic initiative on water cycles (see “On the Web” below).
Over a dozen university researchers will be participating in IHOP through NSF grants alone (see “Just the facts,” page 4). “It’s probably the largest and most expensive project I’ve ever dealt with,” says NSF project officer Stephan Nelson. “It’s also far and away the most complex, in that there are a lot of deployable systems that can be directed to different places depending on the weather. The coordination among those is going to be quite difficult. I think that’ll be one of the biggest challenges of this project.”
As for the often-fickle weather on the Great Plains, Weckwerth is optimistic. ”In the absence of a severe drought, we should be OK.”
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| The IHOP2002 domain extends from southern Kansas to north and west Texas. (Illustration courtesy IHOP and Michael Shibao.) |
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Edited by Bob Henson,
bhenson@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Mon Mar 11 16:42:17 MST 2002